You have to satisfy Bill+Julia = 24

But this is satisfied in both the approaches. Assuming numbers approach: 2+6+0+12 = 20.

Here, I compared 6 with miles Bill ran on sunday when total is 24, i.e. 6/20=x/24 → x=7.2;

Update/new observation: if i change the relation of Bill’s running on Sat & Sun to multiple(like on sun he ran twice he ran on sat) then choosing numbers approach is working fine. I want to know the limitations of choosing numbers technique.